1. Field of the Invention
The present invention relates to a light-emitting element, a light-emitting device, an electronic device, a lighting device, and an organic compound.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Such a light-emitting element is self-luminous elements and has advantages over liquid crystal displays, such as high visibility of the pixels and no need of backlight; thus, such a light-emitting element is thought to be suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed.
Furthermore, since such a light-emitting element can be formed in a film form, the light-emitting element makes it possible to provide planar light emission; thus, a large-area element can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to a lighting device and the like.
Such light-emitting elements utilizing electroluminescence can be broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, application of voltage to the light-emitting element causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus a current flows. The injected electrons and holes then lead the organic compound to its excited state, so that light emission is obtained from the excited organic compound.
The excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state (S*) is called fluorescence, and emission from the triplet excited state (T*) is called phosphorescence. Further, the statistical generation ratio of S* to T* in a light-emitting element is thought to be 1:3.
With a compound that can convert energy of a singlet excited state into light emission (hereinafter, called a fluorescent compound), only light emission from the singlet excited state (fluorescence) is observed and that from the triplet excited state (phosphorescence) is not observed, at room temperature. Accordingly, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
In contrast, with a compound that can convert energy of a triplet excited state into light emission (hereinafter, called a phosphorescent compound), light emission from the triplet excited state (phosphorescence) is observed. Further, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be theoretically increased to 100%. In other words, higher emission efficiency can be obtained than using a fluorescent compound. For this reason, light-emitting elements using a phosphorescent compound have been under active development recently so that high-efficiency light-emitting elements can be achieved.
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound, the light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound as the matrix is called a host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is called a guest material (dopant).
In the case where a phosphorescent compound is a guest material, a host material needs to have higher triplet excited energy (energy difference between a ground state and a triplet excited state) than the phosphorescent compound. The host material also needs to have properties of easily accepting and transporting both holes and electrons (i.e., a bipolar property).
When a substance used as a host material has a bipolar property, holes and electrons can be accepted efficiently; thus, a light-emitting element in which such a host material is used in a light-emitting layer can have lower driving voltage.
Furthermore, since singlet excitation energy (energy difference between a ground state and a singlet excited state) is higher than triplet excited energy, a substance that has high triplet excited energy also has high singlet excitation energy. Therefore the above substance that has high triplet excited energy is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance.
As a host material which has a bipolar property and higher triplet excited energy than a phosphorescent compound, a carbazole derivative which has a heteroaromatic ring having, in the same molecule, a carbazole skeleton with a hole-transport property and an oxadiazole skeleton or a quinoxaline skeleton with an electron-transport property which is the heteroaromatic ring is disclosed (e.g., Patent Document 1).